尝试计算每位会员借阅的平均书籍数量。
会员( memb no ,姓名,年龄)
书( isbn ,标题,作者,出版商)
借( memb no,isbn ,date)
大胆 - 主键
斜体 - 外键
这里有人可以帮帮我吗?在此先感谢
答案 0 :(得分:1)
要获得每个成员借来的不同书籍的数量,你可以做到
select membno, count(*) as totalborrowed
from borrowed
group by membno
要获得会员总数,您可以
select count(distinct membno) as totalmembers
from borrowed
要获得平均借书,您应该将结果合并。与1.0
相乘得到小数结果。
select 1.0 * sum(totalborrowed)/count (distinct b.membno) as avg_per_member
from
(select membno, count(*) as totalborrowed
from borrowed
group by membno) t join borrowed b on t.membno = b.membno
答案 1 :(得分:0)
select AVG(MemberBookCount)
from (
select count(*) * 1.0 as MemberBookCount
from MemberBook
group by Member
) a
答案 2 :(得分:0)
SELECT COUNT (*) / CAST((SELECT COUNT (*) FROM member) AS FLOAT)
FROM borrowed
答案 3 :(得分:0)
我认为最简单的方法是两个值的比率:
select count(*) * 1.0 / count(distinct membo)
from borrowed;